While the capacity of optical communication systems has rapidly increased over the last decade, the modulation technique employed in the majority of realizations has remained binary amplitude shift keying (also referred as on-off key (OOK)) in either nonreturn-to-zero (NPZ) or return-to-zero (RZ) format. There have been employed alternative modulation and demodulation techniques recently in optical communications, such as duobinary, carrier-suppressed return-to-zero (CSRZ), differential phase shift keying (DPSK). In DPSK format, the information is carried by phase change between two adjacent symbols. The phase change is limited to 0 and π in binary DPSK. If the phase change can be 0, π/2, π, 3π/2, it is called as differential quadrature phase shift keying (optical DQPSK). Compared with traditional OOK, DPSK has the advantage of requiring a ˜3 dB optical signal-to-noise ratio (OSNR) gain and robustness to nonlinear effects. Because the optical DQPSK transmits the four-level symbol, it doubles the spectral efficiency, which relaxes the requirements of electrical device speed, optical dispersion management, polarization mode dispersion, and so on. In summary, optical DQPSK is a promising candidate for the next generation optical communication system.
As described in an article “Optical Differential Quadrature Phase-Shift Key (ODQPSK) for High Capacity Optical Transmission” by R. A. Griffin et al., OFC 2002, a typical optical DQPSK receiver consists of a pair of Mach-Zehnder interferometers, corresponding to I branch and Q branch respectively, each with an optical delay τ equal to the symbol period of transmission system. The differential optical phase between interferometer arms is set to π/4 for the I branch interferometer and −π/4 for the Q branch respectively. Two output terminals of the interferometers are connected to balanced optical detectors to recover the transmitted data. The configuration and operation of optical DQPSK Transmitter and optical DQPSK receiver are also described in, for example, PCT application WO2002/051041.
In the receiver, a very important issue is to set the differential optical phase between interferometers arms exactly π/4 and −π/4, otherwise, an excessive OSNR penalty will be caused. To realize this, a control feedback loop is typically employed. It monitors the receiver phase error, and then it generates the phase adjust signal to adjust the receiver phase so that the phase is locked to target value. One typical control feedback method is so called dither-peak-detection method. In this method, the receiver phase is slightly detuned with a fixed frequency f, while monitoring the 2 f component of some kind of error signal. When the receiver phase is locked to target value, the 2 f component of the error signal reaches minimum.
However, the dither-peak-detection method has natural drawbacks, including:
1. The phase detuning will cause excessive OSNR penalty.
2. The peaking detection only provides whether the actual phase is the target value. It cannot provide whether the actual phase is larger or less than the target value.
3. The peak detection signal is usually quadric to the phase error, so that the sensitivity of peak detection signal to the actual phase error reduces when phase error is near zero. As a result, the phase control accuracy is not high.
4. The phase control speed is limited by the dither frequency.
In view of the foregoing, there is an urgent need in the art for novel phase control technique that overcomes the aforementioned shortcomings in an effective, practical way.